Field of the Invention
The present invention relates to a sealed compressor for use in a refrigeration cycle device such as a refrigerator, an air compressor, etc.
Description of the Related Art
In recent years, there has been an increasing demand for energy saving to conserve global environment. In particular, there has been a strong demand for higher efficiency in compressors for use in refrigerators, other refrigeration cycle devices, or the like, air compressors for use in fields of industries, etc.
As a conventional sealed compressor of this type, there is known a compressor in which a recess is formed on the upper surface of a piston reciprocatable inside a cylinder and its efficiency is improved (Japanese Examined Patent Application Publication. No. Hei. 8-6689)
FIG. 22 is a longitudinal sectional view of a conventional sealed compressor disclosed in Japanese Examined Patent Application Publication No. Hei. 8-6689. FIG. 23 is a plan view of a piston of the conventional sealed compressor when viewed from a tip end surface side. FIG. 24 is an enlarged cross-sectional view of major components of the upper portion of the piston and a valve plate portion in the conventional sealed compressor.
Referring to FIGS. 22, 23, and 24, in this sealed compressor, a sealed container 1 reserves oil 2 in a bottom portion thereof and is filled with a working fluid 4. The sealed compressor 1 accommodates a compressor body 6 elastically supported inside the sealed container 1 by a suspension spring 8.
The compressor body 6 includes an electric (electrically driven) element 10 and a compression element 12 rotationally driven by the electric element 10. The compression element 12 is positioned below the electric element 10. The electric element 11 includes a stator 14 and a rotor 16.
The compression element 12 includes a crankshaft 22 having a main shaft 20 and an eccentric shaft 18, a cylinder 26 defining a compression chamber 24, a cylinder block 30 provided integrally with a bearing 28 supporting the main shaft 20, a piston 32 which is slidable inside the cylinder 26, a valve plate 34 for closing the end surface of the cylinder 26, a suction valve 38 provided on the valve plate 34 to open and close a suction hole (not shown) and a discharge hole 36 which provide communication between inside and outside of the compression chamber 24, a discharge valve 40, and a coupling member 42 for coupling the eccentric shaft 18 to the piston 32.
A cylinder head 44 is positioned to cover the valve plate 34 on an opposite side of the compression chamber 24. The valve plate 34 and the cylinder head 44 form a head space 46.
The main shaft 20 of the crankshaft 22 is pivotally mounted on the bearing 28 of the cylinder block 30. The rotor 16 is fastened to the main shaft 20.
As shown in FIGS. 23 and 24, a recess 50 is formed on an upper surface (tip end surface) 48 of the piston 32. When viewed from a direction in which the piston 32 moves, at least a portion of the recess 50 overlaps with a portion of the discharge hole 36. A surface 52 of the upper surface 48 which is other than the recess 50 is flat, and is parallel to an inner surface of the valve plate 34.
The operation of the above configured conventional sealed compressor will now be described.
In the sealed compressor, a current is supplied to the stator 14 to generate a magnetic field, and thereby the rotor 16 secured to the main shaft 20 is rotated, which causes the crankshaft 22 to be rotated. The piston 32 reciprocatingly slides inside the cylinder 26 via the coupling member 42 attached to the eccentric shaft 18. Thus, a series of cycles which are a suction step, a compression step, and a discharge step are repeated.
In the suction step, when the piston 32 moves in a direction to increase the volume of the cylinder 26, the working fluid 4 in the compression chamber 24 is expanded. When the pressure in the compression chamber 24 becomes lower than a suction pressure, the suction valve 38 opens, due to a difference between a pressure in the compression chamber 24 and a pressure in a lower-pressure side (not shown) of a refrigeration cycle. The working fluid 4 which has returned from the refrigeration cycle and has a low temperature flows into the compression chamber 24 through the suction hole (not shown).
Then, in the compression step, when the piston 32 moves from a bottom dead center corresponding to a greatest volume of the compression chamber 24 in a direction to reduce the volume of the compression chamber 24, the pressure in the compression chamber 124 increases, and the suction valve 38 is closed, due to a difference between the pressure in the compression chamber 24 and the pressure in the lower-pressure side (not shown) of the refrigeration cycle, so that the compression chamber 24 is closed.
Thereafter, when the piston 32 moves in a direction to further reduce a volume of the compression chamber 24, the working fluid 4 is compressed up to a predetermined pressure.
In the discharge step, when the pressure of the working fluid 4 inside the compression chamber 24 increases and becomes higher than a pressure in the head space 46 defined by the valve plate 34 and the cylinder head 44, the discharge valve 40 opens due to a pressure difference, causing the working fluid 4 inside the compression chamber 24 to flow into the head space 46 through the discharge hole 36. Then, the working fluid 4 flows into a discharge muffler (not shown) from the head space 46 and is released to a higher-pressure side (not shown) of the refrigeration cycle.
When the piston 32 is in a top dead center in which the piston 32 is positioned closest to the valve plate 34 and the volume of the compression chamber 24 is smallest, there is a clearance between the piston 32 and the valve plate 34 to avoid interference between the piston 32 and the valve plate 34, and there is a small volume left in the compression chamber 24. The working fluid 4 remains in this small volume and is not discharged. Therefore, in the suction step, the remaining working fluid 4 and the working fluid 4 which has newly flowed into the compression chamber 24 through the suction hole (not shown) are mixed and compressed together.
The recess 50 formed on the upper surface 48 of the piston 32 increases a clearance of a space between the valve plate 34 and the recess 50 in a state where the piston 32 is in the top dead center, thereby ensuring a greater area of a fluid passage through which the working fluid 4 moves from the upper surface 48 of the piston 32 across the space between the valve plate 34 and the recess 50 and flows into the discharge hole 36.
As a result, the flow state of the working fluid 4 flowing into the discharge hole 36 can be improved. By reducing a distance of the clearance between the valve plate 34 and the upper surface 48 of the piston 32 and reducing the volume of this space in the state where the piston 32 is in the top dead center, a volume efficiency of the compressor can be improved.
There is also known a configuration in which a projection is provided on the tip end surface of a piston, and the projection moves into a discharge hole of a valve plate, thereby lessening the amount of a working fluid remaining in a compression chamber to a minimum level (see e.g., Japanese Laid-Open Patent Application Publication No. 2010-90705).
However, in the conventional configuration disclosed in Japanese Examined Patent Application Publication No. Hei. 8-6689, in the compression step in which the piston 32 moves in the direction to reduce the volume of the compression chamber 24, the working fluid 4 flows toward the center of the recess 50 in the vicinity of the upper surface 48 and the recess 50 in the piston 32. Therefore, flow components of the working fluid 4 cross each other in the center portion of the recess 50.
This results in a situation in which the flow of the working fluid 4 inside the compression chamber 24 is disordered when the working fluid 4 is compressed, which precludes the flow of the working fluid 4 into the discharge hole 36.
Therefore, in the above configuration, when the piston 32 is in the top dead center, the weight of the working fluid 4 remaining in the space between the piston 32 and the valve plate 34 increases, and the remaining working fluid 4 re-expands in the suction step, which results in a reduced volume efficiency.
In the configuration disclosed in Japanese Laid-Open Patent Application Publication No. 2010-90705, efficiency of the compressor can be possibly improved effectively, but the flow of the working fluid flowing toward the projection is disordered, which leaves a room for improvement.